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58                          ORIGIN OF TOOTH PATTERNS                      CHAP.

" talon"; and this ledge became produced into two additional
cusps, the hypoconule or hypoconulid, and the ectoeone or ecto-
conid. Thus the typical sextuberculate tooth of the primitive
Ungulate, anrl indeed of many primitive Eutherians, is arrived at.

FIG. 39-—Epitome of the evolution of a cusped tooth. 1, Reptile ; 2, DromatJierium •
3, M'icroconodon / 4, Spalacotherium: me, raetacotiid ; jtw, paraconicl; pr, proto-
coiiid ; 5, ATngohithzrium. (After Osborn.)
ITrom this the still further complicated teeth of modern Ungulates
can be derived by further additions or fusions, etc.1 On the other
hand, the development of the Primate molar stops short at the
stage of four cusps.
That such a series can be traced is an undoubted fact. Every
stage exists, or has existed. But whether the stages can be con-
nected or not is quite another question. It is by three main
lines of argument that the view here sketched out in brief is
supported. In the first place, the tracing of the pedigrees of
many groups of mammals has met with very considerable success;
and it is clear that as we pass from the living Horse and
Hhinoceros, with their complicated molars, to their forerunners,
we find that both can be referred to a primitive Ungulate molar
with but six cusps. Going still further back to the lowest
Eocene and ancestral type as it appears, JEuprotogonia, we still
find in the molar tooth the sextubercular plan of structure. We
can. hardly get further back in the evolution of the Perissodactyles
with any probability of security. On the other hand, many facts
point to a fundamental relationship between the primitive Ungu-
lates and the early Creodonts. The latter frequently show plainly
trifcubercular molars. Such Ungulates as JSujprotogonia and Proto-
gonodon, though sex- or quinque-tubercular as to their molars,
have a distinctly prevailing trituberculiani, when the si%e and
importance of three of the cusps is taken into account. But this
* e.g. the "protolopli/' " metaloph," etc.  (see Fig. S6, p. 51), of tl*e modern
form of tooth.